How Many IP Addresses in a /25 Subnet? (128 — Here’s Why)
128 IP addresses.
That’s the answer. A /25 subnet contains exactly 128 IP addresses. But only 126 are actually usable — the first address is the network identifier, and the last is the broadcast address.
A /25 is exactly half of a /24. It’s created by splitting a /24 in two.
Why 128? The Math Behind /25
The “/25” in CIDR notation means 25 bits are used for the network portion of the address. That leaves 7 bits for host addresses.
The calculation:
- 32 bits total in an IPv4 address
- 25 bits for network = 7 bits for hosts
- 2^7 = 128 possible combinations
- Subtract 2 (network + broadcast), you get 126 usable addresses
It’s not arbitrary. It’s binary math.
Complete CIDR Reference Chart
| CIDR | Subnet Mask | Total IPs | Usable IPs | Common Name |
|---|---|---|---|---|
| /25 | 255.255.255.128 | 128 | 126 | Half /24 |
| /24 | 255.255.255.0 | 256 | 254 | Standard block |
What Can You Actually Do With 126 IPs?
Typical /25 allocation: Small business office with ~100 devices on one VLAN, branch office with ~80 workstations + servers, or small hosting company with ~90 customer VMs.
Summary
- /25 = 128 IPs (126 usable)
- Exactly half of a /24
- Subnet mask: 255.255.255.128
- Calculation: 2^(32-25) = 128
- Common use: Splitting /24 for network segmentation
